The guardhouse can be reached only by the Warden-Commander during the final part of the Smuggler's Run side-quest. Assassinating the Lieutenant of the City Guard will provide the key to open up Jacen's cell. The key can also be stolen.
Once the cell is open, Jacen will recognize and praise the Commander and he will offer his archery skills as a reward for the latter's rescue. If the Warden-Commander agrees to it, Jacen can participate in The Siege of Vigil's Keep by sniping targets.
If the Commander refuses Jacen's offer, the latter will simply bid them farewell and escape the guardhouse.If the Warden-Commander happens to be an archer, the Warden can ask Jacen if he's a better archer than them. Jacen will then nervously respond by saying he was the best archer in the arling, up until recently.